The present invention relates to a color image forming apparatus in which a toner image is formed on an image forming body by means of electrophotography and the toner image is transferred onto a transfer sheet so that the image can be obtained, and more particularly relates to a color image forming apparatus such as a printer, copier or facsimile in which a plurality of charging means, exposure means and developing means are provided around the image forming body and a color image can be obtained when the image forming body is rotated by one revolution.
A large number of methods and apparatus have been conventionally proposed to form a color image by means of electrophotography. From the viewpoint of the accuracy of the color register and the resolution of the color image, techniques disclosed in the official gazettes of Japanese Patent Publication Open to Public Inspection Nos. 75850/1985, 76766/1985, 95456/1985 are excellent.
According to the aforementioned techniques, a color image can be obtained in such a manner that: a latent image forming process according to the number of separated colors of a document image and a developing process are repeatedly conducted on a drum-shaped image forming body so that toner images of each color can be superimposed on the surface of the aforementioned image forming body; the color toner image then transferred onto a transfer sheet; and the transferred image is fixed. In the aforementioned techniques, only one optical laser device is used for an exposing means to form a latent image in accordance with the number of separated colors, and a color image is formed on the drum-shaped image forming body when it is rotated a plurality of times.
Instead of the drum-shaped image forming body described in the aforementioned official gazettes, a belt-shaped image forming body has been disclosed in the official gazette of Japanese Patent Publication Open to Public Inspection No. 102261/1987, in which a photoconductive material is coated or vapor-deposited on a flexible belt.
The belt-shaped image forming body is provided between rotating rollers including a drive roller, so that space is effectively utilized and the apparatus can be made compact. Accordingly, the belt-shaped image forming body is very effective when a color printer, for example, is designed.
However, in order to form a color image on the belt-shaped image forming body using only one optical laser system provided on the periphery of the belt-shaped image forming body, it is necessary to rotate the aforementioned belt-shaped image forming body a number of times, which increases the time and cost of the copying operation.
In order to solve this conventional problem, a color image forming apparatus can be considered which is characterized in that: a plurality of optical laser units are disposed in parallel below the belt-shaped image forming body; and a color image is formed by one revolution of the belt-shaped image forming body. According to the aforementioned color image forming apparatus, it is not necessary to rotate the belt-shaped image forming body a plurality of times, so that a color image can be formed like that of a black and white image. Consequently, the aforementioned apparatus is advantageous in that: the copying time is shortened; the life of the apparatus is prolonged; and the apparatus can be operated easily.
However, the following problems are caused in the aforementioned color image forming apparatus.
(1) A wide space is necessary to install a plurality of optical laser systems, so that the size of the apparatus becomes large. PA0 (2) In order to form a color image, a high accuracy of color register is required. In the case of the aforementioned apparatus, optical laser systems are dependently provided, so that adjusting operations such as positioning for writing-in timing by scanning light, positioning of each optical laser unit, parallelism of scanning light, and adjustment of scanning width, become very complicated, so that it is actually difficult to conduct adjustment of high accuracy.
In order to form a color image by means of electrophotography, a method has been disclosed in the official gazettes of Japanese Patent Publication Open to Public Inspection Nos. 75850/1985, 76766/1985, 95456/1985, 95458/1985, 158475/1985, wherein a color image can be obtained by a method in which: a latent image forming process according to the number of separated colors of a document image and developing process are repeatedly conducted on a drum-shaped image forming body so that toner images of each color can be superimposed on the surface of the image forming body; the color toner image is transferred onto a transfer sheet; and the transferred image is fixed.
There are two examples of this method. The first example is a color image forming apparatus in which exposing and developing units are provided around the belt-shaped image forming body in accordance with the number of separated colors (for example, three colors of yellow, magenta and cyan, or four colors including black). The second example is a color image forming apparatus in which exposing and developing units are provided around a drum-shaped image forming body in accordance with the number of separated colors. The following are explanations for the first example in which the belt-shaped image forming body is provided.
The color image forming apparatus is composed in such a manner that a belt-shaped image forming body made of a flexible belt on which a photoconductive material is coated or vapor-deposited, encircles a plurality of rollers including a tension roller so that tension can be given. The belt-shaped image forming body is rotated, maintaining slidable contact with a guide member so that the surface of the belt-shaped image forming body can be maintained at a predetermined position. The color image forming apparatus comprises a charger; an exposing unit; and a plurality of developing units, in which color toners of different colors are housed, disposed around the aforementioned belt-shaped image forming body, wherein a predetermined gap is formed between the image forming means and the rotating belt-shaped image forming body.
In the color image forming apparatus, the following method can be considered: a register mark is formed on the image forming body; the aforementioned register mark is detected by a sensor; and exposure operations are sequentially conducted by a plurality of exposing units according to the results of detection so that latent image formation can be started from the same position on the aforementioned image forming body.
In the aforementioned color image forming apparatus, the register mark formed on the surface of the belt-shaped image forming body is read by a single or a plurality of sensors so that the timing to start exposure can be determined. In this manner, it is necessary to accurately set the distance between the sensor and the exposing unit, and the distances between the plurality of exposing units, wherein the accuracy must be .+-.0.01-0.1 mm. However, when the apparatus is put into practical use, it is hard to maintain the aforementioned accuracy; for example, the accuracy is generally limited to .+-.0.3-0.5 mm using conventional methods. Further, after the exposing unit has been once removed from the apparatus, it is extremely difficult to position it in exactly the same position.
In the aforementioned color image forming apparatus in which toner images are superimposed on the surface of the image forming body, the following problems are caused: unless the distance between the plurality of exposing units and the aforementioned belt-shaped image forming body is set accurately (i.e., the accuracy is one pixel unit, which typically is not more than 80 .mu.m), the quality of the color toner image is deteriorated. Positioning control in the belt-shaped image forming body is more difficult than that in the drum-shaped image forming body.